Yes, the room service is free, but you'll need to tip the person that brings it up. You should also be able to order any dinner that they are serving in the dining room that evening, including dessert :-)
i love it when cruisers say that room service is free, no its not free, there is just no extra charge for room service except for the tip The reason why ,is that you have already pre paid for your food. Remember you paid for a cruise that includes food lodging and entertainment.
It is true that Carnival has a limited menu, but let me tell you where it was so helpful and enjoyable. On the ship, there were cards to hang on the door for breakfast/roomservice. We would fill this out at night, hang it on the door, and let them wake us up in the morning, with fresh coffee, danish, juice etc. You have to tell them what time you want it delivered, so if we wanted to get up at 8:00 we would tell them 8:00. Now your spouse might greet you sometimes with coffee, but what about for the both of you every morning. It was wonderful. You will hear all kinds of things about cruising, and usually someone is finding fault, but for every fault, there is good. Keep that in mind, and have a great trip.